Brain (1992), 115, 1081-1092
TEXTURE DISCRIMINATION IN CARPAL TUNNEL SYNDROME by J. T . HEYWOOD 1 and
J . w.
MORLEY 2
(From the 'Department of Clinical Neurophysiology, The Prince Henry Hospital and the 2School of Physiology and Pharmacology, University of New South Wales, Sydney, Australia)
SUMMARY The ability to discriminate textured surfaces was measured in patients with carpal tunnel syndrome (CTS). Patients were initially diagnosed using clinical and electrophysiological criteria. The textured surfaces were gratings of alternating ridges and grooves. The gratings differed in their spatial period only, with the ratio of the ridge width to the groove width remaining constant (1:5). A two-alternative forced-choice paradigm was employed in which subjects, both patients and age-matched controls, rubbed their index finger (D2) or little finger (D5) back and forth across the surfaces. The grating spatial period at which subjects could discriminate a difference between the standard grating (spatial period = 2000 /tm) and the comparison grating (spatial period in the range 2000—2900 /*m) with a probability of 0.75 was taken as the measure of discriminative ability. Statistical comparison of the mean 75% values showed that: (i) when patients used D2 their discriminative ability was significantly impaired in comparison with the discriminative ability of controls using either D2 or D5; (ii) there was no significant difference in discriminative ability between patients and controls when patients used D5 to discriminate the textures; (iii) the 75% values for patients using D2 or D5 did not differ significantly. The degree of abnormality of each patient's sensory evoked potential did not allow us to predict their subsequent performance on the discrimination task.
INTRODUCTION
The carpal tunnel syndrome (CTS) results from the chronic compression of the median nerve as it passes through the carpal tunnel, a narrow space formed by the carpal bones and the flexor retinaculum. The syndrome predominantly affects middle-aged individuals and is conventionally diagnosed by a combination of clinical and electrophysiological criteria. Symptoms are initially sensory, with paraesthesiae and numbness in the distribution of the median nerve, and pain which may be felt in the hand or arm. These symptoms characteristically wax and wane and are usually more severe at night, disturbing sleep. For a full review of the clinical features of CTS, see Sunderland (1978). The median nerve is easily studied using electrophysiological techniques, as the nerve trunk is in a superficial position both proximal and distal to the carpel tunnel. The ulnar nerve runs parallel to the median nerve at the wrist, but it lies outside the carpal tunnel, and thus may be used as a control. Slowing of conduction velocity and/or loss of amplitude of the sensory or motor evoked potential across the carpal tunnel segment of the median nerve is regarded as reliable evidence of CTS (Simpson, 1956; Gilliatt and Sears, 1958; Thomas, 1960; Thomas et al., 1967). Marin et al. (1983) have reported that the sensitivity Correspondence to: Dr J. W. Morley, School of Physiology and Pharmacology, University of New South Wales, PO Box 1, Kensington, NSW 2003, Australia. © Oxford University Press 1992
1082
J. T. HEYWOOD AND J. W. MORLEY
of these parameters in the detection of CTS is further increased following provocative manoeuvres such as flexion of the wrist. A number of studies have been carried out to look at the pattern of sensory loss in electrophysiologically confirmed cases of CTS, using both threshold and discriminative measures of sensation. No matter which sensory measure was chosen, only a proportion of patients in these studies had impaired sensation in the distribution of the median nerve. The proportion varied between studies. For example, the reported sensitivity of vibration threshold as a test for sensory loss in CTS has varied from 4% to 87 %, while the reported sensitivity of static two-point discrimination has varied from 18% to 38% (Gelberman et al., 1980; Szabo et al., 1984; Borg and Lindblom, 1988; Merchut et al., 1990). Vibration thresholds have also been reported to increase in patients with CTS, but not in control subjects, following provocation of the CTS symptoms with wrist flexion (Borg and Lindblom, 1986). The equivocal nature of these results throws some doubt on the usefulness of simple tactile stimuli in the assessment of sensory loss in CTS. Our aim in this study was to investigate sensory dysfunction in patients with CTS using tactile stimuli that is more complex than simple vibration or two point discrimination (Borg and Lindblom, 1988; Merchut et al., 1990). We chose to use grating surfaces of alternating ridges and grooves as our test stimulus. Gratings are a simple texture and have advantages over other textured surfaces, such as sandpaper, in that they can be accurately defined by their spatial period (the distance from the leading edge of one ridge to the leading edge of the following ridge) and the ratio of the ridge width to the groove width. The spatial characteristics can be precisely varied in one dimension, thereby allowing a graded series of textured surfaces to be produced, each differing in only one spatial parameter. The ability of normal subjects to disciminate incremental changes in the spatial parameters of grating surfaces has been previously investigated (Morley et al, 1983).
METHODS Texture discrimination was studied in two groups of subjects, a patient group and a control group. The patient group consisted of 20 subjects, each of whom satisfied our criteria for CTS (see below). The age range of the patient group was 33—62 yrs, with a mean age of 46 yrs. Fifteen (75%) of the patients were female. Subjects included in this group were selected from patients referred to the Department of Clinical Neurophysiology, The Prince Henry Hospital, for electrodiagnostic assessment. The control group consisted of 18 subjects selected from hospital staff and the community. The age range of the control group was 26—62 yrs, with a mean age of 44 yrs. Thirteen (72%) of the control subjects were female. No hospital patients were included in the control group. All subjects gave informed consent to participate in the study. Criteria for inclusion in patient group For the purposes of this study, subjects were defined as having CTS, and therefore included in the patient group, if they satisfied all of the following criteria: (i) the presence of symptoms characteristic of CTS; (ii) an abnormal median nerve sensory conduction potential; (iii) an ulner nerve sensory conduction potential that was within normal limits; (iv) no history that was suggestive of peripheral neuropathy. The symptoms considered to be characteristic of CTS were the presence of numbness, paraesthesiae or pain involving the index finger and/or the middle finger, with symptoms waking the patient at night or present on waking in the morning. Patients were not included if they described sensory symptoms in the lower limbs, of if they suffered from diseases (heavy alcohol intake, hypothyroidism, renal disease, diabetes, rheumatoid arthritis) that are associated with peripheral neuropathy.
TEXTURE DISCRIMINATION IN CTS
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Nerve conduction testing As evoked sensory nerve potentials have been reported to be a more sensitive indicator of early CTS (Kemble, 1968; Kopell and Goodgold, 1968; Buchthal et al., 1974; Cioni et al., 1989), compound motor potentials were not used in defining CTS in the patient group. Orthodromic sensory nerve action potentials (SNAPs) were evoked by stimulating the digital nerves of the index finger (D2) or little finger (D5) using ring electrodes wrapped around the base and the proximal phalanx of the digit. Sensory nerve action potentials were recorded at the wrist using bipolar Ag/AgCl surface electrodes placed over the median nerve (D2) or the ulnar nerve (D5). The distance between recording electrodes was 4 cm and the ground electrode was placed on the palm. Constant-voltage rectangular pulses of 0.1 ms duration were delivered at 1 Hz, with stimulus intensity suprathreshold. The surface temperature of the skin was measured at the base of D2 and at the wrist, and was maintained at >32° C. Evoked potentials were averaged using a Medelec electromyograph (MS6 or MS92B). Latency was measured from the onset of the stimulus artefact to the onset of the negative phase of the SNAP. The distance between the cathode and the active (distal) recording electrode was divided by the latency to determine conduction velocity. For the purposes of this study, the SNAP for the median nerve (orthodromic D2 to wrist) was considered to be abnormal only if the evoked potential was: (i) less than 1 /tV peak-to-peak in amplitude or (ii) more than 5 m/s slower than the corresponding ulnar nerve SNAP. The SNAP for the ulnar nerve (orthodromic D5 to wrist) was judged to be within normal limits where: (i) the conduction velocity was greater than 50 m/s and (ii) the SNAP amplitude was greater than 5 #iV peak-to-peak (Cioni et al., 1989). Palmar stimulation of the median nerve was also performed. In patients with CTS this technique would be expected to demonstrate a focal slowing of conduction velocity in the segment of nerve between palm and wrist. Recording electrodes were placed distally around D2 and over the median nerve at the wrist. A bipolar stimulating electrode was held in the mid-palm, with the cathode located between the first and second metacarpals. Conduction velocity from palm to finger (antidromic conduction) was compared with that from palm to wrist (orthodromic conduction). A recent study in normal subjects has shown that the conduction velocity of the two segments should be within 2 m/s (P. J. Goadsby and D. Burke, personal communication). Palmar stimulation ensured that all patients enrolled in the study were suffering from CTS and not from a median nerve injury located more distally in the hand. Further clinical assessment of patients A hand-held pin was used to test for impaired fingertip sensation. The patient was asked whether the sensation associated with stimulation of the fingertip of D2 and D5 felt the same. Sensation in D2 was considered abnormal if the patient reported a reduced sensation in D2. Two-point discrimination was measured on D2 and D5 using a pair of blunt calipers applied transversely across the fingerpad. The smallest distance between the caliper points for which the patient could discern the existence of two points was recorded in millimetres, and was considered abnormal if the value for D2 was greater than that for D5. Measurement of two-point discrimination was carried out as it is considered by some authors to be a sensitive test of sensory loss due to CTS (Borg and Lindblom, 1988). We compared the ability of patients to discriminate the two-point stimulus with their ability to discriminate the textured surfaces used in this study to assess the relative sensitivity of each discrimination test in detecting carpal tunnel compression. Two provocative tests were performed: Tinel's sign (paraesthesiae radiating into the hand and fingers when the median nerve was percussed gently over the wrist with a tendon hammer) and Phalen's wrist flexion text (initiating or aggravation of the patient's usual symptoms when the wrist was passively flexed for 1 min). Assessment of control subjects Control subjects were screened for evidence of CTS. They were included in the control group if they had: (i) no symptoms (pain or paraesthesiae) in the forearm, hand or fingers; (ii) no symptoms or disease (heavy alcohol intake, hypothyroidism, renal disease, diabetes, rheumatoid arthritis) that are associated with peripheral neuropathy; (iii) normal median and ulnar nerve SNAP. The median nerve SNAP was considered to be normal if the conduction velocity was greater than 50 m/s and amplitude greater than 10 ^V peak-to-peak. Palmar stimulation was not performed on the control subjects.
1084
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Texture discrimination The textured surfaces used in this study were gratings consisting of alternating ridges and grooves. The grating surfaces were 10 cm long and 4 cm wide and were manufactured from nylon using a photopolymerization technique commonly used in letterpress printing. The gratings were accurately defined by the spatial period (distance from the leading edge of one ridge to the leading edge of the following ridge) and by the ratio of the ridge width to groove width (see Morley et al., 1983). Ten different grating sizes were employed in which the spatial period of gratings increased in 100 /un increments from 2000 /xm to 2900 /im. The ridge : groove ratio remained constant at 1:5. Texture discrimination in D2 and D5 was tested in the affected hand for subjects in the patient group and in the preferred hand of control subjects. Each trial consisted of a pair of grating surfaces being presented. The left-hand grating was the same in each trial and had a spatial period of 2000 /*m (standard) while the spatial period of the second grating of the pair (comparison) was in the range 2000—2900 /im. The comparison surfaces were presented in a pseudo-random order. Testing alternated between D2 and D5; 25 pairs of surfaces were examined using one finger, and then the other finger was tested. In most cases each pair of surfaces was presented 10 times to each finger. If the subject gave an identical response to the first five presentations of a pair, then that pair was no longer presented using that finger, diereby reducing testing time. The discrimination task was a two-alternative forced-choice procedure. Subjects were required to rub their fingerpad, either D2 or D5, over each surface of a pair and respond same or different. Subjects could use any movement profile and fingerpad force that they felt best optimized their discrimination. The number of times the subject responded different was counted and converted to a percentage of the total number of presentations of that particular surface pair. This percentage figure corresponds, for each pair of surfaces, to the probability of the subject giving the correct response (except for the one pair of surfaces that was not different, 2000-2000 urn). A screen prevented the subjects seeing the surfaces and white noise from a loudspeaker masked any sound generated by the subjects rubbing their fingerpad over the grating. Each testing session lasted approximately 45 min.
RESULTS
Measurement of sensory nerve conduction velocity Median and ulnar nerve sensory conduction velocities were measured in both patients and controls. The range of median nerve conduction velocities measured from D2 in patients was 26—55 m/s (mean 39 m/s, SD =7.4, the two patients with zero amplitude D2 SNAPs were not included) and in controls was 54—65 m/s (mean 59 m/s, SD=3.8). Ulnar nerve conduction velocities in patients and controls were very similar, with a range of 51 - 7 6 m/s in patients (mean 60 m/s, SD=6.2) and 5 0 - 6 7 m/s in controls (mean 57 m/s, SD=5.1). An ulnar nerve conduction velocity of 50 m/s or greater was our criterion for a normal ulnar nerve SNAP, based on the findings of Cioni et al. (1989). Cioni et al. studied 56 normal hands and found that their conduction velocities ranged from 49 m/s to 68 m/s, with a mean velocity of 55.9 m/s and SD = 4.0. This gives a mean minus 2 SDs of 47.9 m/s which compares favourably with the 20 normal hands of our study. Palmar stimulation was performed on 18 patients. In 16 of these patients, conduction velocity from the palm to the wrist was more than 8 m/s slower than the conduction velocity from D2 to the palm, confirming that there was focal slowing of conduction across the wrist segment of the median nerve. In the remaining two patients no response could be recorded with palmar stimulation. These patients both had a very small median nerve SNAP at the wrist ( < 1 /xV peak-to-peak).
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Increment in spatial period (jim) Fie. 1. Discrimination functions for four patients. The number of times each comparison grating was called different from the standard grating (expressed as a percentage of the total number of presentations of that grating size) is plotted against the increment in the spatial period of the comparison grating relative to the spatial period of the standard grating (i.e. 2000 /un). The discrimination function for D2 is shown by squares and a solid line and for D5 by circles and a dashed line.
Texture discrimination Discrimination functions for four patients are shown in Fig. 1. The graphs are plots of the number of times each comparison grating was called different from the standard grating (expressed as a percentage of the total number of presentations for that comparison grating) against the increment in the spatial period of the comparison grating relative to the standard. The data for D2 are shown by squares and solid line and for D5 by circles and dashed line. The graphs for each patient show that as the comparison surface size increased there was an increase in discriminative performance. The discrimination functions for D2 are also shifted to the right relative to the functions for D5, indicating a deterioration in performance when D2 was used to discriminate the surfaces. The mean discrimination functions for both D2 and D5 in patients is shown in Fig. 2A and for controls in Fig. 2B. The mean function for D2 of patients is shifted to the right relative to the function for D5, while for the controls there is very little difference between the discrimination functions for D2 and D5. Comparison of the D2 functions for patients and controls indicates that there is a marked difference between the discriminative
J. T. HEYWOOD AND J. W. MORLEY
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Increment in spatial period FIG. 2. Mean discrimination functions of patients (A) and controls (B). The format is the same as for Fig. 1 except that the ordinate is the mean of the number of times each comparison grating was called different from the standard grating (expressed as a percentage) for all patients or controls. Discrimination function for D2 is shown by squares and a solid line and for D5 by circles and a dashed line. Error bars are ± 1 standard error of the mean.
performance of the patients and controls, with the patients displaying an impaired performance over the majority of the range of surface sizes tested. Measure of discriminative performance While the discriminative behaviour of both patients and control subjects is described by the complete discrimination function, it is useful to extract a single measure that is representative of this capacity. A measure that is commonly used is the grating spatial period that yields a probability of correct response of 0.75. This represents a value halfway between a response of 50%, where the subject performs no better than chance, and 100%, where the subject can discriminate between the two surfaces without error. This value is analogous to the difference limen of the standard psychometric function (McNicol, 1972; Morley et al., 1983). We calculated the 75% correct value for each subject using linear interpolation between data points on either side of 75 %. One patient, who did not reach 75% correct, has not been included in the analysis. The 75% values for patients and controls using D2 and D5 are presented in Fig. 3. The mean 75% value for patients using D2 was 2480 /im (SD= 157) and using D5 was 2424 urn (SD = 196). Mean values for controls using D2 was 2285 jim (SD= 119) and using D5 was 2333 /an (SD = 117). Statistical comparison (t test with Bonferroni correction; Wallenstein et al., 1980) of the mean 75% values of patients and controls showed that: (i) when patients used D2 their discriminative performance was significantly worse than that of controls using either D2 (P < 0.01) or D5 (P < 0.05); (ii) the performance of the patients and controls did not differ significantly (P > 0.05) when both groups of subjects used D5; (iii) the 75% values for patients using D2 or D5 did not differ significantly (P > 0.05).
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TEXTURE DISCRIMINATION IN CARPAL TUNNEL SYNDROME by J. T . HEYWOOD 1 and
J . w.
MORLEY 2
(From the 'Department of Clinical Neurophysiology, The Prince Henry Hospital and the 2School of Physiology and Pharmacology, University of New South Wales, Sydney, Australia)
SUMMARY The ability to discriminate textured surfaces was measured in patients with carpal tunnel syndrome (CTS). Patients were initially diagnosed using clinical and electrophysiological criteria. The textured surfaces were gratings of alternating ridges and grooves. The gratings differed in their spatial period only, with the ratio of the ridge width to the groove width remaining constant (1:5). A two-alternative forced-choice paradigm was employed in which subjects, both patients and age-matched controls, rubbed their index finger (D2) or little finger (D5) back and forth across the surfaces. The grating spatial period at which subjects could discriminate a difference between the standard grating (spatial period = 2000 /tm) and the comparison grating (spatial period in the range 2000—2900 /*m) with a probability of 0.75 was taken as the measure of discriminative ability. Statistical comparison of the mean 75% values showed that: (i) when patients used D2 their discriminative ability was significantly impaired in comparison with the discriminative ability of controls using either D2 or D5; (ii) there was no significant difference in discriminative ability between patients and controls when patients used D5 to discriminate the textures; (iii) the 75% values for patients using D2 or D5 did not differ significantly. The degree of abnormality of each patient's sensory evoked potential did not allow us to predict their subsequent performance on the discrimination task.
INTRODUCTION
The carpal tunnel syndrome (CTS) results from the chronic compression of the median nerve as it passes through the carpal tunnel, a narrow space formed by the carpal bones and the flexor retinaculum. The syndrome predominantly affects middle-aged individuals and is conventionally diagnosed by a combination of clinical and electrophysiological criteria. Symptoms are initially sensory, with paraesthesiae and numbness in the distribution of the median nerve, and pain which may be felt in the hand or arm. These symptoms characteristically wax and wane and are usually more severe at night, disturbing sleep. For a full review of the clinical features of CTS, see Sunderland (1978). The median nerve is easily studied using electrophysiological techniques, as the nerve trunk is in a superficial position both proximal and distal to the carpel tunnel. The ulnar nerve runs parallel to the median nerve at the wrist, but it lies outside the carpal tunnel, and thus may be used as a control. Slowing of conduction velocity and/or loss of amplitude of the sensory or motor evoked potential across the carpal tunnel segment of the median nerve is regarded as reliable evidence of CTS (Simpson, 1956; Gilliatt and Sears, 1958; Thomas, 1960; Thomas et al., 1967). Marin et al. (1983) have reported that the sensitivity Correspondence to: Dr J. W. Morley, School of Physiology and Pharmacology, University of New South Wales, PO Box 1, Kensington, NSW 2003, Australia. © Oxford University Press 1992
1082
J. T. HEYWOOD AND J. W. MORLEY
of these parameters in the detection of CTS is further increased following provocative manoeuvres such as flexion of the wrist. A number of studies have been carried out to look at the pattern of sensory loss in electrophysiologically confirmed cases of CTS, using both threshold and discriminative measures of sensation. No matter which sensory measure was chosen, only a proportion of patients in these studies had impaired sensation in the distribution of the median nerve. The proportion varied between studies. For example, the reported sensitivity of vibration threshold as a test for sensory loss in CTS has varied from 4% to 87 %, while the reported sensitivity of static two-point discrimination has varied from 18% to 38% (Gelberman et al., 1980; Szabo et al., 1984; Borg and Lindblom, 1988; Merchut et al., 1990). Vibration thresholds have also been reported to increase in patients with CTS, but not in control subjects, following provocation of the CTS symptoms with wrist flexion (Borg and Lindblom, 1986). The equivocal nature of these results throws some doubt on the usefulness of simple tactile stimuli in the assessment of sensory loss in CTS. Our aim in this study was to investigate sensory dysfunction in patients with CTS using tactile stimuli that is more complex than simple vibration or two point discrimination (Borg and Lindblom, 1988; Merchut et al., 1990). We chose to use grating surfaces of alternating ridges and grooves as our test stimulus. Gratings are a simple texture and have advantages over other textured surfaces, such as sandpaper, in that they can be accurately defined by their spatial period (the distance from the leading edge of one ridge to the leading edge of the following ridge) and the ratio of the ridge width to the groove width. The spatial characteristics can be precisely varied in one dimension, thereby allowing a graded series of textured surfaces to be produced, each differing in only one spatial parameter. The ability of normal subjects to disciminate incremental changes in the spatial parameters of grating surfaces has been previously investigated (Morley et al, 1983).
METHODS Texture discrimination was studied in two groups of subjects, a patient group and a control group. The patient group consisted of 20 subjects, each of whom satisfied our criteria for CTS (see below). The age range of the patient group was 33—62 yrs, with a mean age of 46 yrs. Fifteen (75%) of the patients were female. Subjects included in this group were selected from patients referred to the Department of Clinical Neurophysiology, The Prince Henry Hospital, for electrodiagnostic assessment. The control group consisted of 18 subjects selected from hospital staff and the community. The age range of the control group was 26—62 yrs, with a mean age of 44 yrs. Thirteen (72%) of the control subjects were female. No hospital patients were included in the control group. All subjects gave informed consent to participate in the study. Criteria for inclusion in patient group For the purposes of this study, subjects were defined as having CTS, and therefore included in the patient group, if they satisfied all of the following criteria: (i) the presence of symptoms characteristic of CTS; (ii) an abnormal median nerve sensory conduction potential; (iii) an ulner nerve sensory conduction potential that was within normal limits; (iv) no history that was suggestive of peripheral neuropathy. The symptoms considered to be characteristic of CTS were the presence of numbness, paraesthesiae or pain involving the index finger and/or the middle finger, with symptoms waking the patient at night or present on waking in the morning. Patients were not included if they described sensory symptoms in the lower limbs, of if they suffered from diseases (heavy alcohol intake, hypothyroidism, renal disease, diabetes, rheumatoid arthritis) that are associated with peripheral neuropathy.
TEXTURE DISCRIMINATION IN CTS
1083
Nerve conduction testing As evoked sensory nerve potentials have been reported to be a more sensitive indicator of early CTS (Kemble, 1968; Kopell and Goodgold, 1968; Buchthal et al., 1974; Cioni et al., 1989), compound motor potentials were not used in defining CTS in the patient group. Orthodromic sensory nerve action potentials (SNAPs) were evoked by stimulating the digital nerves of the index finger (D2) or little finger (D5) using ring electrodes wrapped around the base and the proximal phalanx of the digit. Sensory nerve action potentials were recorded at the wrist using bipolar Ag/AgCl surface electrodes placed over the median nerve (D2) or the ulnar nerve (D5). The distance between recording electrodes was 4 cm and the ground electrode was placed on the palm. Constant-voltage rectangular pulses of 0.1 ms duration were delivered at 1 Hz, with stimulus intensity suprathreshold. The surface temperature of the skin was measured at the base of D2 and at the wrist, and was maintained at >32° C. Evoked potentials were averaged using a Medelec electromyograph (MS6 or MS92B). Latency was measured from the onset of the stimulus artefact to the onset of the negative phase of the SNAP. The distance between the cathode and the active (distal) recording electrode was divided by the latency to determine conduction velocity. For the purposes of this study, the SNAP for the median nerve (orthodromic D2 to wrist) was considered to be abnormal only if the evoked potential was: (i) less than 1 /tV peak-to-peak in amplitude or (ii) more than 5 m/s slower than the corresponding ulnar nerve SNAP. The SNAP for the ulnar nerve (orthodromic D5 to wrist) was judged to be within normal limits where: (i) the conduction velocity was greater than 50 m/s and (ii) the SNAP amplitude was greater than 5 #iV peak-to-peak (Cioni et al., 1989). Palmar stimulation of the median nerve was also performed. In patients with CTS this technique would be expected to demonstrate a focal slowing of conduction velocity in the segment of nerve between palm and wrist. Recording electrodes were placed distally around D2 and over the median nerve at the wrist. A bipolar stimulating electrode was held in the mid-palm, with the cathode located between the first and second metacarpals. Conduction velocity from palm to finger (antidromic conduction) was compared with that from palm to wrist (orthodromic conduction). A recent study in normal subjects has shown that the conduction velocity of the two segments should be within 2 m/s (P. J. Goadsby and D. Burke, personal communication). Palmar stimulation ensured that all patients enrolled in the study were suffering from CTS and not from a median nerve injury located more distally in the hand. Further clinical assessment of patients A hand-held pin was used to test for impaired fingertip sensation. The patient was asked whether the sensation associated with stimulation of the fingertip of D2 and D5 felt the same. Sensation in D2 was considered abnormal if the patient reported a reduced sensation in D2. Two-point discrimination was measured on D2 and D5 using a pair of blunt calipers applied transversely across the fingerpad. The smallest distance between the caliper points for which the patient could discern the existence of two points was recorded in millimetres, and was considered abnormal if the value for D2 was greater than that for D5. Measurement of two-point discrimination was carried out as it is considered by some authors to be a sensitive test of sensory loss due to CTS (Borg and Lindblom, 1988). We compared the ability of patients to discriminate the two-point stimulus with their ability to discriminate the textured surfaces used in this study to assess the relative sensitivity of each discrimination test in detecting carpal tunnel compression. Two provocative tests were performed: Tinel's sign (paraesthesiae radiating into the hand and fingers when the median nerve was percussed gently over the wrist with a tendon hammer) and Phalen's wrist flexion text (initiating or aggravation of the patient's usual symptoms when the wrist was passively flexed for 1 min). Assessment of control subjects Control subjects were screened for evidence of CTS. They were included in the control group if they had: (i) no symptoms (pain or paraesthesiae) in the forearm, hand or fingers; (ii) no symptoms or disease (heavy alcohol intake, hypothyroidism, renal disease, diabetes, rheumatoid arthritis) that are associated with peripheral neuropathy; (iii) normal median and ulnar nerve SNAP. The median nerve SNAP was considered to be normal if the conduction velocity was greater than 50 m/s and amplitude greater than 10 ^V peak-to-peak. Palmar stimulation was not performed on the control subjects.
1084
J. T. HEYWOOD AND J. W. MORLEY
Texture discrimination The textured surfaces used in this study were gratings consisting of alternating ridges and grooves. The grating surfaces were 10 cm long and 4 cm wide and were manufactured from nylon using a photopolymerization technique commonly used in letterpress printing. The gratings were accurately defined by the spatial period (distance from the leading edge of one ridge to the leading edge of the following ridge) and by the ratio of the ridge width to groove width (see Morley et al., 1983). Ten different grating sizes were employed in which the spatial period of gratings increased in 100 /un increments from 2000 /xm to 2900 /im. The ridge : groove ratio remained constant at 1:5. Texture discrimination in D2 and D5 was tested in the affected hand for subjects in the patient group and in the preferred hand of control subjects. Each trial consisted of a pair of grating surfaces being presented. The left-hand grating was the same in each trial and had a spatial period of 2000 /*m (standard) while the spatial period of the second grating of the pair (comparison) was in the range 2000—2900 /im. The comparison surfaces were presented in a pseudo-random order. Testing alternated between D2 and D5; 25 pairs of surfaces were examined using one finger, and then the other finger was tested. In most cases each pair of surfaces was presented 10 times to each finger. If the subject gave an identical response to the first five presentations of a pair, then that pair was no longer presented using that finger, diereby reducing testing time. The discrimination task was a two-alternative forced-choice procedure. Subjects were required to rub their fingerpad, either D2 or D5, over each surface of a pair and respond same or different. Subjects could use any movement profile and fingerpad force that they felt best optimized their discrimination. The number of times the subject responded different was counted and converted to a percentage of the total number of presentations of that particular surface pair. This percentage figure corresponds, for each pair of surfaces, to the probability of the subject giving the correct response (except for the one pair of surfaces that was not different, 2000-2000 urn). A screen prevented the subjects seeing the surfaces and white noise from a loudspeaker masked any sound generated by the subjects rubbing their fingerpad over the grating. Each testing session lasted approximately 45 min.
RESULTS
Measurement of sensory nerve conduction velocity Median and ulnar nerve sensory conduction velocities were measured in both patients and controls. The range of median nerve conduction velocities measured from D2 in patients was 26—55 m/s (mean 39 m/s, SD =7.4, the two patients with zero amplitude D2 SNAPs were not included) and in controls was 54—65 m/s (mean 59 m/s, SD=3.8). Ulnar nerve conduction velocities in patients and controls were very similar, with a range of 51 - 7 6 m/s in patients (mean 60 m/s, SD=6.2) and 5 0 - 6 7 m/s in controls (mean 57 m/s, SD=5.1). An ulnar nerve conduction velocity of 50 m/s or greater was our criterion for a normal ulnar nerve SNAP, based on the findings of Cioni et al. (1989). Cioni et al. studied 56 normal hands and found that their conduction velocities ranged from 49 m/s to 68 m/s, with a mean velocity of 55.9 m/s and SD = 4.0. This gives a mean minus 2 SDs of 47.9 m/s which compares favourably with the 20 normal hands of our study. Palmar stimulation was performed on 18 patients. In 16 of these patients, conduction velocity from the palm to the wrist was more than 8 m/s slower than the conduction velocity from D2 to the palm, confirming that there was focal slowing of conduction across the wrist segment of the median nerve. In the remaining two patients no response could be recorded with palmar stimulation. These patients both had a very small median nerve SNAP at the wrist ( < 1 /xV peak-to-peak).
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Increment in spatial period (jim) Fie. 1. Discrimination functions for four patients. The number of times each comparison grating was called different from the standard grating (expressed as a percentage of the total number of presentations of that grating size) is plotted against the increment in the spatial period of the comparison grating relative to the spatial period of the standard grating (i.e. 2000 /un). The discrimination function for D2 is shown by squares and a solid line and for D5 by circles and a dashed line.
Texture discrimination Discrimination functions for four patients are shown in Fig. 1. The graphs are plots of the number of times each comparison grating was called different from the standard grating (expressed as a percentage of the total number of presentations for that comparison grating) against the increment in the spatial period of the comparison grating relative to the standard. The data for D2 are shown by squares and solid line and for D5 by circles and dashed line. The graphs for each patient show that as the comparison surface size increased there was an increase in discriminative performance. The discrimination functions for D2 are also shifted to the right relative to the functions for D5, indicating a deterioration in performance when D2 was used to discriminate the surfaces. The mean discrimination functions for both D2 and D5 in patients is shown in Fig. 2A and for controls in Fig. 2B. The mean function for D2 of patients is shifted to the right relative to the function for D5, while for the controls there is very little difference between the discrimination functions for D2 and D5. Comparison of the D2 functions for patients and controls indicates that there is a marked difference between the discriminative
J. T. HEYWOOD AND J. W. MORLEY
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Increment in spatial period FIG. 2. Mean discrimination functions of patients (A) and controls (B). The format is the same as for Fig. 1 except that the ordinate is the mean of the number of times each comparison grating was called different from the standard grating (expressed as a percentage) for all patients or controls. Discrimination function for D2 is shown by squares and a solid line and for D5 by circles and a dashed line. Error bars are ± 1 standard error of the mean.
performance of the patients and controls, with the patients displaying an impaired performance over the majority of the range of surface sizes tested. Measure of discriminative performance While the discriminative behaviour of both patients and control subjects is described by the complete discrimination function, it is useful to extract a single measure that is representative of this capacity. A measure that is commonly used is the grating spatial period that yields a probability of correct response of 0.75. This represents a value halfway between a response of 50%, where the subject performs no better than chance, and 100%, where the subject can discriminate between the two surfaces without error. This value is analogous to the difference limen of the standard psychometric function (McNicol, 1972; Morley et al., 1983). We calculated the 75% correct value for each subject using linear interpolation between data points on either side of 75 %. One patient, who did not reach 75% correct, has not been included in the analysis. The 75% values for patients and controls using D2 and D5 are presented in Fig. 3. The mean 75% value for patients using D2 was 2480 /im (SD= 157) and using D5 was 2424 urn (SD = 196). Mean values for controls using D2 was 2285 jim (SD= 119) and using D5 was 2333 /an (SD = 117). Statistical comparison (t test with Bonferroni correction; Wallenstein et al., 1980) of the mean 75% values of patients and controls showed that: (i) when patients used D2 their discriminative performance was significantly worse than that of controls using either D2 (P < 0.01) or D5 (P < 0.05); (ii) the performance of the patients and controls did not differ significantly (P > 0.05) when both groups of subjects used D5; (iii) the 75% values for patients using D2 or D5 did not differ significantly (P > 0.05).
TEXTURE DISCRIMINATION IN CTS JUUU
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